3 hours should be fine, especially if you're flying on a major airline, since they fly domestic flights out of the same terminal as international flights (T3). Only LCCs (SpiceJet, Indigo, GoAir) use T1.
If you do need to transfer, figure on 1 hour for customs and immigration, and assume you need to check in one hour early, which leaves you an hour -- sufficient, but doesn't leave you a whole lot of buffer if something/anything goes wrong. Shuttle buses run every 20 minutes, or if you're in a pinch you can take a taxi (~15 min).
In any case, if it's not too late, I'd recommend booking both your international and domestic flights on the same ticket, that way you're covered if you miss the connection. Fog regularly causes total chaos at DEL, especially in winter.